Ion guiding devices are widely employed in mass spectrometers to transport ions efficiently, and without loss, through the different regions of the instrument. For instance, ion guides may be used to transport ions between various regions of different pressures, e.g. from high or atmospheric pressures in the source region into the high vacuum stages of the instrument containing the analyser (typically operating at pressures of about 10−5 to 10−9 mbar).
One known type of ion guide is a so-called stacked ring ion guide (“SRIG”) comprising a plurality of axially stacked electrodes each having an aperture formed therein through which ions are transmitted in use. SRIG devices can be constructed relatively inexpensively, simply by slotting the electrodes into their axial positions on a suitable holder.
Furthermore, because the electrodes are axially stacked and spaced apart from each other, SRIG devices allow the possibility of selectively applying different DC potentials to each of the electrodes such that axial fields can be applied across a portion of the device. For instance, this allows the implementation of travelling wave techniques, where ions are driven along the length of the ion guide by translating a series of axial potential wells along the ion guide, in order to increase the speed of transfer of ions through these regions. Travelling wave techniques are particularly advantageous for clearing ions from an ion guide quickly, as the ions can be translated along the ion guide without requiring high DC gradients that may take a significant time to stabilise after being ramped and/or may introduce unwanted ion activation in the downstream components.
In a SRIG device alternate RF phases are applied to adjacent electrodes (i.e. +−+−) in order to confine the ions radially, but only one RF phase (i.e. + or −) is applied to each of the electrodes.
Another known type of ion guide is a quadrupole ion guide comprising a set of four parallel rods arranged in a quadrilateral array, with adjacent rods being connected to alternate RF phases and opposite rods connected to the same RF phase. Thus, both RF phases (+ and −) must be present at each axial position along the length of the quadrupole ion guide. The resulting quadrupole field generally provides better focussing, i.e. focusses ions closer to the central axis, than a SRIG device. Quadrupole ion guides may therefore allow the use of smaller differential apertures between different vacuum stages, which may in turn allow for the use of smaller, less expensive pumps. Alternatively, quadrupole ion guides may allow more ions to be focussed through an aperture of a given size.
However, the voltage requirements for quadrupoles are much higher than with SRIGs, especially for larger r0 values, where quadrupoles require much higher voltages than equivalently sized SRIGs, and so the effect of variations in frequency, and interference may be more significant. This can lead to difficulties in terms of providing both phases of the RF voltages to the rods without breakdown or interference. Quadrupoles must therefore generally be manufactured with a high amount of precision, and are typically harder and more expensive to manufacture and maintain than SRIG devices. Furthermore, it is difficult to implement travelling waves on a quadrupole rod set. Although segmented rod sets are known, which allow axial DC gradients to be applied along the length of the device, typically adjacent segments of the rod set are still coupled, e.g. to form a resistive network, and the axial segments are not independent of each other.
It is therefore desired to provide an improved ion guiding device.